Measuring devices, which are implemented for progressive tracking of a target point and a coordinative position determination of this point, can generally, in particular in conjunction with industrial surveying, be summarized under the term laser trackers. A target point can be represented in this case by a retroreflective unit (for example, a cube prism), which is targeted using an optical measurement beam of the measuring device, in particular a laser beam. The laser beam is reflected in parallel back to the measuring device, wherein the reflected beam is captured using a capture unit of the device. An emission or reception direction of the beam is ascertained in this case, for example, by means of sensors for angle measurement, which are associated with a deflection mirror or a targeting unit of the system. In addition, a distance from the measuring device to the target point is ascertained with the capture of the beam, for example, by means of runtime or phase difference measurement or by means of the Fizeau principle.
In addition, in modern tracker systems, a deviation of the received measurement laser beam from a so-called servo-monitoring point is ascertained on a sensor—increasingly as a standard feature. By means of this measurable deviation, a position difference between the center of a retroreflector and the point of incidence of the laser beam on the reflector can be determined and the alignment of the laser beam can be corrected or tracked as a function of this deviation such that the deviation on the sensor is decreased, in particular is “zero”, and therefore the beam is aligned in the direction of the reflector center. By way of the tracking of the laser beam alignment, progressive target tracking (tracking) of the target point can be performed and the distance and position of the target point can be progressively determined in relation to the tracker system. The tracking can be implemented in this case by means of an alignment change of the deflection mirror, which is movable by a motor, provided for deflecting the laser beam and/or by a pivot of the targeting unit, which has the beam-guiding laser optic.
The described target tracking must be preceded by locking of the laser beam on the reflector. For this purpose, a capture unit having a position-sensitive sensor and having a comparatively large field of vision can additionally be arranged on the tracker. In addition, in devices of the type in question, additional illumination means are integrated, using which the target or the reflector is illuminated, in particular using a defined wavelength differing from the wavelength of the distance measuring means. The sensor can be implemented in this context to be sensitive to a range around this specific wavelength, for example, to reduce or entirely prevent external light influences. By means of the illumination means, the target can be illuminated and, using the camera, an image of the target having illuminated reflector can be captured. By way of the depiction of the specific (wavelength-specific) reflection on the sensor, the reflection position in the image can be resolved and therefore an angle in relation to the capture direction of the camera and a direction to the target or reflector can be determined. An embodiment of a laser tracker having such a target search unit is known, for example, from WO 2010/148525 A1. In dependence on the direction information thus derivable, the alignment of the measurement laser beam can be changed such that a distance between the laser beam and the reflector, onto which the laser beam is to be locked, is decreased.
Laser trackers of the prior art have at least one distance meter for distance measurement, wherein it can be implemented as an interferometer, for example. Since such distance measuring units can only measure relative distance changes, so-called absolute distance meters are installed in addition to interferometers in current laser trackers. For example, such a combination of measuring means for distance determination is known by way of the product AT901 of Leica Geosystems AG. Furthermore, a combination of an absolute distance meter and an interferometer for distance determination using a HeNe laser is known, for example, from WO 2007/079600 A1.
Laser trackers according to the prior art can additionally be embodied having an optical image capture unit having a two-dimensional, light-sensitive array, for example, a CCD or CID camera or a camera based on a CMOS array, or having a pixel array sensor and having an image processing unit. The laser tracker and the camera can be installed one on top of another in this case, in particular in such a manner that the positions thereof in relation to one another are not variable. The camera is, for example, rotatable together with the laser tracker about its essentially perpendicular axis, but is pivotable up-and-down independently of the laser tracker and is therefore arranged separately from the optic of the laser beam in particular. Furthermore, the camera—for example, in dependence on the respective application—can be embodied as pivotable about only one axis. In alternative embodiments, the camera can be installed in an integrated construction together with the laser optic in a shared housing.
With the capture and analysis of an image—by means of image capture and image processing unit—of a so-called measuring aid instrument or measuring aid object having markings, the relative locations of which to one another are known, an orientation of the instrument and of an object (for example, a probe), which is arranged on the measuring aid instrument, in space can be concluded. Together with the determined spatial position of the target point, furthermore the position and orientation of the object in space can be precisely determined absolutely and/or in relation to the laser tracker (6DoF determination: determination of six degrees of freedom).
Such measuring aid instruments can be embodied by so-called scanning tools, which are positioned having the contact point thereof on a point of the target object. The scanning tool has markings, for example, light-emitting diodes (LEDs), and a reflector, which represents a target point on the scanning tool and can be targeted using the laser beam of the tracker, wherein the positions of the markings and the reflector in relation to the contact point of the scanning tool are precisely known. The measuring aid object can also be, in a way known to a person skilled in the art, a handheld scanner equipped for distance measurement, for example, for contactless surface surveying, wherein the direction and position of the scanner measurement beam used for the distance measurement are precisely known in relation to the light-emitting diodes and reflectors which are arranged on the scanner. Such a scanner is described, for example, in EP 0 553 266.
An image having the captured and well (i.e., in particular completely) imaged light-emitting diodes and preferably a known arrangement of the light-emitting diodes on the object is used for a reliable determination of the spatial orientation (6DoF measurement) of the measuring aid object. The orientation of the measuring aid object in space can be derived therefrom by means of image analysis.
In such a 6DoF measurement, a spatial resection is calculated with the aid of the known geometry and with knowledge of the internal orientation of the camera and the orientation of the measuring aid object is determined therefrom. The result of the spatial resection becomes less precise, however (i.e., random measurement uncertainty increases), if individual LEDs are at least partially not visible to the camera.
A particularly unfavorable case occurs if one or more of the LEDs are partially concealed, so that a calculated image coordinate in the captured image is systematically corrupted for a light spot of a respective LED. As a consequence, with the corruption of individual image coordinates, the result of the resection and therefore the determination of the spatial orientation of the measuring aid object are systematically corrupted.